Sub-floor smoothing compounds, or underlayments, are cement-based materials generally fluid in use but set and harden to form a hard, absorbent, smooth surface suitable to receive floor coverings. They can be used to smooth out imperfections in a sub-floor which would otherwise be mirrored through the final floor finish. They may also be used as a thin screed, by the addition of aggregate, to provide a level floor where the necessary thickness for a conventional cement and sand screed is not available. There are two main types of underlayment; powder and water-based, or powder and latex -based compounds.